Polyacrylic acid (salt)-type water absorbent resin and method for producing of same

ABSTRACT

A process for producing a water absorbent resin is provided with which it is possible to efficiently and inexpensively obtain a surface-crosslinked water-absorbing resin having excellent material properties, while ensuring high productivity. In continuous production scaled up to a large scale (in particular, 1 t/hr or more), the physical properties are improved and stabilized (for example, a reduction in standard deviation of the physical properties) by a surface crosslinking treatment to further improve absorption against pressure (AAP) and saline flow conductivity (SFC). The process for producing a polyacrylic acid (salt)-type water absorbent resin includes a surface treatment step in which after addition of a surface-crosslinking agent, a crosslinking reaction is conducted in a transverse type continuous stirring apparatus having stirring means including a feeding inlet and a discharging outlet of a water absorbent resin and one or more rotary shafts having with a plurality of stirring discs, and heating means, at a stirring-power index of the device of 3-15 W·hr/kg. 
       (Stirring-power index)=((power consumption of apparatus at the time surface treatment)−(power consumption at the time of idling))×average retention time)/(treatment amount per unit time×average retention time)

TECHNICAL FIELD

The present invention relates to a polyacrylic acid (salt)-type water absorbent resin and a method for producing the same. More particularly, the present invention relates to a polyacrylic acid (salt)-type water absorbent resin having high water absorption rate (CRC), high water absorption against pressure (AAP), and high liquid permeability (SFC) and containing little water Extractables and a method for producing the same.

BACKGROUND ART

A water absorbent resin (Super Absorbent Polymer; abbreviated as SAP) has been used in a wide range of uses for sanitary materials such as paper diapers, sanitary napkins, incontinence products for adults, and the like, and uses for water retention agent for soil, owing to properties that the resin can absorb a large quantity of a water-based liquid several times to several hundred times as much as the mass of itself and has been manufactured and consumed in large quantities.

In general, a water absorbent resin is produced by polymerizing an aqueous solution containing a hydrophilic monomer and a crosslinking agent to obtain a hydrous gel-like polymer, drying the gel polymer, and surface-crosslinking the dried product. The physical properties such as water absorption against pressure (AAP) and liquid permeability (GBP, SFC) of the above-mentioned water absorbent resin are improved by surface-crosslinking step. The surface-crosslinking step is commonly a step of providing a highly crosslinked layer in the vicinity of the water absorbent resin surface by causing reaction of the water absorbent resin with a surface-crosslinking agent or a polymerizable monomer.

Various kinds of surface-crosslinking agents reactive on a functional group of a water absorbent resin (particularly, carboxyl group) are proposed as a surface-reforming method of such a water absorbent resin and examples known as the surface-crosslinking agents are oxazoline compounds (Patent Document 1), vinyl ether compounds (Patent Document 2), epoxy compounds (Patent Document 3), oxetane compounds (Patent Document 4), polyhydric alcohol compounds (Patent Document 5), polyamide polyamine-epihalo adducts (Patent Documents 6, 7), hydroxyacrylamide compounds (Patent Document 8), oxazolidinone compounds (Patent Documents 9, 10), bis- or poly-oxazoline compounds (Patent Document 11), 2-oxotetrahydro-1,3-oxazolidine compounds (Patent Document 12), alkylene carbonate compounds (Patent Document 13), and the like. A technique using a specified surface-crosslinking agent (Patent Document 14) is also known.

Techniques also known as the surface-reforming method other than the method carried out by a surface-crosslinking agent are a technique of surface-crosslinking by polymerizing a monomer in the vicinity of the water absorbent resin surface (Patent Document 15) and techniques of radical crosslinking with persulfuric acid salts etc. (Patent Documents 16, 17). Techniques of reforming water absorbent resins by heating without using a surface-crosslinking agent (Patent Documents 18, 19), which is different from common surface-crosslinking treatment, are also known.

A technique of using an additive in combination for mixing a surface-crosslinking agent is also proposed and examples known as the additive are water-soluble cations such as aluminum salts and the like (Patent Documents 20, 21), alkali (Patent Document 22), organic acids and inorganic acids (Patent Document 23), peroxides (Patent Document 24), and surfactants (Patent Document 25).

Not only the chemical methods but also many surface treatment methods using apparatuses and reaction conditions have been proposed. Examples known as a method using an apparatus are techniques using a specified mixing apparatus as a mixing apparatus for a surface-crosslinking agent (Patent Documents 26 to 29) and techniques using a heating apparatus for causing reaction of a water absorbent resin and a surface-crosslinking agent (Patent Documents 30, 31) and the like.

There is also a technique for controlling an increase in heating temperature for causing reaction of a water absorbent resin and a surface-crosslinking agent (Patent Document 32) in improvement of the reaction condition aspect. In a heating step, techniques known are a technique of carrying out surface-crosslinking twice (Patent Document 33), a technique of controlling particle size by drying a water absorbent resin, thereafter carrying out a second heat drying step, and further carrying out surface-crosslinking (Patent Document 34), a technique of defining oxygen partial pressure (Patent Document 35), techniques of defining the spraying conditions and dew points (Patent Documents 37, 38), techniques of defining the mixing conditions of treatment liquids (Patent Documents 39, 40), and a technique paying attention to a cooling step (Patent Document 41).

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: U.S. Pat. No. 6,297,319 -   Patent Document 2: U.S. Pat. No. 6,372,852 -   Patent Document 3: U.S. Pat. No. 6,265,488 -   Patent Document 4: U.S. Pat. No. 6,809,158 -   Patent Document 5: U.S. Pat. No. 4,734,478 -   Patent Document 6: U.S. Pat. No. 4,755,562 -   Patent Document 7: U.S. Pat. No. 4,824,901 -   Patent Document 8: U.S. Pat. No. 6,239,230 -   Patent Document 9: U.S. Pat. No. 6,559,239 -   Patent Document 10: U.S. Pat. No. 6,503,979 -   Patent Document 11: U.S. Pat. No. 6,472,478 -   Patent Document 12: U.S. Pat. No. 6,657,015 -   Patent Document 13: U.S. Pat. No. 5,409,771 -   Patent Document 14: U.S. Pat. No. 5,422,405 -   Patent Document 15: US Patent Application Publication No.     2005/048221 -   Patent Document 16: U.S. Pat. No. 4,783,510 -   Patent Document 17: EP Patent No. 1824910 -   Patent Document 18: U.S. Pat. No. 5,206,205 -   Patent Document 19: EP Patent No. 0603292 -   Patent Document 20: U.S. Pat. No. 6,605,673 -   Patent Document 21: U.S. Pat. No. 6,620,899 -   Patent Document 22: U.S. Pat. No. 7,312,278 -   Patent Document 23: U.S. Pat. No. 5,610,208 -   Patent Document 24: US Patent Application Publication No.     2007/078231 -   Patent Document 25: US Patent Application Publication No.     2005/029352 -   Patent Document 26: U.S. Pat. No. 5,140,076 -   Patent Document 27: U.S. Pat. No. 6,071,976 -   Patent Document 28: US Patent Application Publication No.     2004/240316 -   Patent Document 29: WO No. 2007/065840 pamphlet -   Patent Document 30: US Patent Application Publication No.     2007/149760 -   Patent Document 31: Japan Patent Application Publication No.     2004-352941 -   Patent Document 32: U.S. Pat. No. 6,514,615 -   Patent Document 33: U.S. Pat. No. 5,672,633 -   Patent Document 34: WO No. 2009/028568 pamphlet -   Patent Document 35: US Patent Application Publication No.     2007/0293632 -   Patent Document 36: U.S. Pat. No. 6,720,389 -   Patent Document 37: U.S. Pat. No. 7,183,456 -   Patent Document 38: US Patent Application Publication No.     2007/161759 -   Patent Document 39: US Patent Application Publication No.     2006/057389 -   Patent Document 40: EP Patent No. 0534228 -   Patent Document 41: U.S. Pat. No. 7,378,453

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

It is difficult to satisfy demands from users for physical properties such as water absorption against pressure and liquid permeability of a water absorbent resin only by a surface-crosslinking technique, although there have been proposed many of surface-crosslinking agents (see Patent Documents 1 to 13) and their combination use (see Patent Document 14), auxiliary agents for surface-crosslinking (see Patent Documents 20 to 25), their mixing apparatuses (see Patent Documents 26 to 29) and heating apparatuses (Patent Documents 30, 31), and also various kinds of conditions (see Patent Documents 32 to 41). Along with change of a surface-crosslinking agent and use of a new auxiliary agent, it may be sometimes accompanied with an increase in cost, a decrease in safety, deterioration of other physical properties (e.g., coloration), and the like in some cases. Although causing an effect to a certain extent in a small scale or batch type production in an experimental laboratory level, the above-mentioned means may not sometimes show so much effective in an industrial scale (e.g., 1 t or more per unit hour) such as large scale continuous production as compared with that in a small scale.

The present invention has been completed in terms of the problems, and an object of the present invention is to provide a method for producing a water absorbent resin which is excellent in physical properties and surface-crosslinked efficiently at a low cost while assuring high productivity.

Solutions to the Problems

The inventors of the present invention have made investigations on a surface-crosslinking step for solving the above-mentioned problems and consequently have solved the problems by carrying out heating treatment with a specified apparatus and with also a specified stirring power index in the surface treatment step after addition of a surface-crosslinking agent.

That is, the present invention provide a method for producing a polyacrylic acid (salt)-type water absorbent resin, comprising a step of preparing an aqueous monomer solution of an acrylic acid (salt), a step of continuously polymerizing the aqueous monomer solution, a step of finely shredding a hydrous gel-like crosslinked polymer during or after polymerization, a step of drying the obtained particulate hydrous gel-like crosslinked polymer, and a surface treatment step of adding and reacting a surface treatment agent to and with the dried water absorbent resin powder, wherein crosslinking reaction is carried out at a stirring power index of 3 to 15 W·hr/kg in a transverse type continuous stirring apparatus having stirring means including a feeding inlet and a discharging outlet of a water absorbent resin and one or more rotary shafts having a plurality of stirring discs, and heating means after the addition of the surface-crosslinking agent in the surface treatment step.

Herein, the stirring power index is defined as (stirring power index)=((power consumption of apparatus at the time surface treatment)−(power consumption at the time of idling))×average retention time/(treatment amount per unit time×average retention time).

Effects of the Invention

According to the present invention, in continuous production in a large industrial scale (particularly, treatment amount of 1 t/hr or more), the physical properties (e.g., water absorption against pressure and liquid permeability) can be improved after surface-crosslinking and the fluctuation of physical property (standard deviation) can be narrowed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing one example of the configuration of a heating apparatus or a cooling apparatus having a biaxial transverse type continuous stirring apparatus and being employed in the embodiment of the present invention.

FIG. 2 is a cross-sectional view showing one example of a stirring disk (equipped with scraping blades) of a biaxial heating apparatus or cooling apparatus.

FIG. 3 is a schematic view of the vertical cross section of a heating apparatus and a cooling apparatus having a connected biaxial transverse type continuous stirring apparatus. Herein, the heating apparatus or the cooling apparatus is an apparatus having a similar shape with the same inner area (inner volume) and can correspond to conventional techniques.

FIG. 4 is a cross-sectional view showing one example of a high speed rotation stirring type mixing apparatus usable for mixing a surface-crosslinking agent. Reference numeral 2 represents an inner wall, 6 represents a stirring shaft, and 7 (7 a, 7 b) represents stirring blades.

FIG. 5 is a cross-sectional view showing one example of a transverse type high speed rotation stirring type mixing apparatus usable for mixing a surface-crosslinking agent. Reference numeral 2 represents an inner wall, 6 represents a stirring shaft, and 7 (7 a, 7 b) represents stirring blades.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the polyacrylic acid (salt)-type water absorbent resin of the present invention and a method for producing the same will be described in detail; however, the scope of the present invention is not restricted to the following description, and those other than the following examples can be properly modified and carried out in a range where the gist of the present invention is not impaired. Specifically, the present invention is not limited to each of the following embodiments, and various modifications can be made within a range shown by the claims and embodiments carried out by properly combining each technical means disclosed with different embodiments are also included within the technical scope of the present invention.

[1] DEFINITION OF TERMS (a) “Water Absorbent Resin”

The “water absorbent resin” means water-swelling and water-insoluble “polymer gelling agent” and includes those having the following physical properties. That is, those having, as the water-swelling property, a water absorption rate under no pressure (CRC) of 5 g/g or more. CRC is preferably 10 to 100 g/g and more preferably 20 to 80 g/g. Because of water-insolubility, it is required that water Extractables are in amount of 0 to 50 mass %. The water Extractables are preferably in amount of 0 to 30 mass %, more preferably in amount of 0 to 20 mass %, and even preferably in amount of 0 to 10 mass %.

The water absorbent resin is not limited to be embodiments of 100 mass % of a polymer and may contain other additives (described below) to the extent of retaining the above-mentioned characteristics. That is, even a water absorbent resin composition having the water absorbent resin and additives is generally named as a water absorbent resin in the present invention. The content of the polyacrylic acid (salt)-type water absorbent resin is preferably 70 to 99.9 mass % relative to the entire water absorbent resin, more preferably 80 to 99.7 mass %, and still more preferably 90 to 99.5 mass %. The components other than the water absorbent resin are preferably water in terms of the water absorption speed and impact resistance of powder (particles) and may include, if necessary, additives described below.

(b) “Polyacrylic Acid (Salt)”

The “polyacrylic acid (salt)” means a polymer containing arbitrarily a graft component and, as a repeating unit, an acrylic acid (salt) as a main component. The acrylic acid (salt) as a monomer excluding a crosslinking agent is in an amount of essentially 50 to 100% by mole, preferably 70 to 100% by mole, more preferably 90 to 100% by mole, and still more preferably substantially 100% by mole. The acrylic acid salt as the polymer essentially contains a polyacrylic acid salt and preferably contains a monovalent salt, more preferably an alkali metal salt or ammonium salt, still more preferably an alkali metal salt, and particularly preferably sodium salt. The shape is not particularly limited; however, the polyacrylic acid (salt) is preferably particles or a powder.

(c) “EDANA” and “ERT”

“EDANA” is an abbreviation of European Disposables and Nonwovens Associations. “ERT” is an abbreviation of measurement method (ERT/EDANA Recommended Test Method) of a water absorbent resin on the basis of European Standards (almost Global Standards) as defined below. In this specification, unless otherwise specified, the physical properties of a water absorbent resin are measured based on ERT original text (Known Document: revised in 2002).

(c-1) CRC (ERT441.2-02)

The “CRC” is an abbreviation for Centrifuge Retention Capacity and means water absorption rate under no pressure (simply sometimes referred to as “water absorption rate”). Specifically, the CRC is the water absorption rate (unit; g/g) after 0.200 g of a water absorbent resin in a nonwoven fabric bag is freely swollen in 0.9 mass % saline solution for 30 minutes and dewatered by a centrifuge at 250 G.

(c-2) AAP (ERT442.2-02)

The “AAP” is an abbreviation for Absorption Against Pressure and means water absorption against pressure. Specifically, the APP is the water absorption rate (unit; g/g) after 0.900 g of a water absorbent resin is swollen in 0.9 mass % saline solution for 1 hour under 1.9 kPa load. In the present invention and examples, the measurement is carried out at 4.8 kPa.

(c-3) “Extractables” (ERT 470.2-02)

“Extractables” means the amount of water soluble components (dissolve amount). Specifically, measurement is carried out by adding 1.000 g of the water absorbent resin to 200 ml of an 0.9 mass % aqueous saline solution, stirring the solution for 16 hours, and measuring the amount of a dissolved polymer by pH titration (unit: mass %).

(c-4) Residual monomers (ERT410.2-02)

The “residual monomers” means the amount of monomers remaining in a water absorbent resin. Specifically, the amount of monomers is a value (unit; ppm by mass) obtained by measuring, after 1.000 g of a water absorbent resin is charged to 200 cm³ of 0.9 mass % saline solution and the resultant is stirred for 2 hours, the amount of monomers eluted in the aqueous solution by using high-pressure liquid chromatography.

(c-5) PSD (ERT420.2-02)

The “PSD” is an abbreviation for Particle Size Distribution and means the particle size distribution measured by sieving classification. The mass average particle diameter and the particle diameter distribution width can be measured by the same method as in “(1) Average Particle Diameter and Distribution of Particle Diameter” described in European Patent No. 0349240, p. 7, lines 25-43 and WO 2004/069915.

(c-6) Others

“pH” (ERT400.2-02): The “pH” means pH of a water absorbent resin.

“Moisture Content” (ERT 430.2-02) The moisture content means the water content of a water absorbent resin.

“Flow Rate” (ERT 450.2-02) The flow rate means the flow down speed of a water absorbent resin powder.

“Density” (ERT 460.2-02): The density means the bulk specific density of a water absorbent resin.

(d) “Liquid Permeability”

The “liquid permeability” means the flow of a liquid flowing among particles of swollen gel under a load or no load. The “liquid permeability” can be measured by SFC (Saline Flow Conductivity) or GBP (Gel Bed Permeability) as a representative measurement method.

The “SFC” is liquid permeability of 0.69 mass % physiological saline solution in a water absorbent resin at a load of 0.3 psi. It is measured according to an SFC testing method described in U.S. Pat. No. 5,669,894.

The “GBP” is liquid permeability of 0.69 mass % physiological saline solution in a water absorbent resin under a load or free expansion. It is measured according to a GBP testing method described in WO 2005/016393 pamphlet.

(e) “Standard Deviation”

The “standard deviation” is a numeral value showing the degree of dispersion of data and means a positive square root of the value calculated by totalizing the square value of the difference of the value of data of n samples and their arithmetic average, that is, the deviation, and dividing the total by n−1. It is used for understanding the degree of fluctuation for the phenomenon with considerable fluctuation. In this specification, the standard deviation is employed for digitalization of the fluctuation (deflection) for a desired physical value of interest.

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(f) Others

In this specification, “X to Y” showing a range means “X or more and Y or lower”. Additionally, the unit of mass “t (ton)” means “Metric ton”. Further, the measurement of physical properties of a water absorbent resin is carried out under the conditions of a temperature of 20 to 25° C. (sometimes simply referred to as “room temperature” or “normal temperature”) and a relative humidity of 40 to 50%, unless otherwise stated.

[2] A METHOD FOR PRODUCING POLYACRYLIC ACID (SALT)-TYPE WATER Absorbent Resin (1) Polymerization Step (a) Monomer (Excluding a Crosslinking Agent)

A monomer of the present invention contains the above-mentioned acrylic acid or its salt as a main component and in terms of water absorption characteristics and decrease of the residual monomers, the acid groups of a polymer are preferable to be neutralized and the neutralization ratio is 10 to 100% by mole preferable, more preferably 30 to 95% by mole, still more preferably 50 to 90% by mole, and particularly preferably 60 to 80% by mole, The neutralization may be carried out for the polymer (hydrogel) after polymerization or for the monomer; however, in terms of productivity and improvement of AAP, neutralization of the monomer is preferable. Consequently, the monomer in the present invention includes a partially neutralized salt of the acrylic acid.

Further, in the present invention, a hydrophilic or hydrophobic unsaturated monomer may be used besides an acrylic acid (salt). Monomers usable may include methacrylic acid, maleic acid, maleic anhydride, 2-(meth)acrylamido-2-methylpropanesulfonic acid, (meth)acryloxyalkanesulfonic acid, N-vinyl-2-pyrrolidone, N-vinylacetamide, (meth)acrylamide, N-isopropyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, polyethylene glycol (meth)acrylate, stearyl acrylate, and their salts.

(b) Crosslinking Agent (Inner Crosslinking Agent)

In the present invention, in terms of the water absorbent properties, use of a crosslinking agent (i.e.; inner crosslinking agent) is particularly preferable. The crosslinking agent is used in an amount of preferably 0.001 to 5% by mole, more preferably 0.005 to 2% by mole, still more preferably 0.01 to 1% by mole, and particularly more preferably 0.03 to 0.5% by mole to the monomer excluding the crosslinking agent, in terms of physical aspect.

Examples usable as the crosslinking agent are one or more of polymerizable crosslinking agents (with polymerizable double bond of the acrylic acid), reactive crosslinking agents (with a carboxyl group of the monomer), and crosslinking agents having both of these properties. Concrete examples are, as a polymerizable crosslinking agent, compounds having at least two polymerizable double bonds in a molecule such as N,N′-methylenebisacrylamide, (poly)ethylene glycol di(meth)acrylate, (polyoxyethylene)trimethylolpropane tri(meth)acrylate, poly(meth)allyloxyalkanes, etc. Further, examples of the reactive crosslinking agent are covalent-binding crosslinking agents such as polyglycidyl ether (ethylene glycol diglycidyl ether or the like), poly alcohols (propanediol, glycerin, sorbitol, etc.), and ion-binding crosslinking agents such as polyvalent metal compounds of aluminum or the like. Among these crosslinking agents, in terms of water absorbent properties, polymerizable crosslinking agents (with the acrylic acid), particularly, acrylate type, allyl type, and acrylamide type polymerizable crosslinking agents are preferably used.

(c) Neutralizing Salt

Preferable examples as a basic substance to be used for neutralization of the acrylic acid may include monovalent bases such as alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide etc., and alkali metal (hydrogen) carbonates such as sodium (hydrogen) carbonate, potassium (hydrogen) carbonate, etc. Particularly, in terms of decrease of the residual monomers, neutralization into an alkali metal acrylate especially with sodium hydroxide is preferable. The preferable conditions or the like in these neutralization treatments are exemplified in International Publication No. 2006/522181 and the disclosed conditions are applicable for the present invention. The neutralization temperature is preferably in a range of 10 to 100° C., more preferably in a range of 30 to 90° C. The neutralization temperature is properly determined in this range, and a neutralization method described below is preferable to decrease the residual monomers.

(d) Concentration of Monomer

Monomers may be polymerized generally in an aqueous solution. The solid content is generally 10 to 90 mass %, preferably 20 to 80 mass %, more preferably 30 to 70 mass %, and particularly preferably 35 to 60 mass %. The polymerization may be carried out in the form of a slurry (water dispersion liquid) exceeding the saturated concentration; however: in terms of physical properties, it is carried out in an aqueous solution with the saturated concentration or lower.

(e) Other Monomer Components

The aqueous unsaturated monomer solution may contain a water-soluble resin or a water absorbent resin such as starch, polyacrylic acid (salt), or polyethylene imine, in combination with a monomer, in an amount of, for example, 0 to 50 mass %, preferably 0 to 20 mass %, particularly preferably 0 to 10 mass %, and most preferably 0 to 3 mass %. The solution may further contain a various kinds of foaming agents (carbonates, azo compounds, air bubbles, etc.), surfactants, or additives described below, in an amount of, for example, 0 to 5 mass % and preferably 0 to 1 mass % to improve the various physical properties of a water absorbent resin or particulate water absorbent agent to be obtained. A graft polymer obtained by using other components (e.g., starch-acrylic acid graft polymer) or a water absorbent resin composition is also generically referred to as a polyacrylic acid (salt)-type water absorbent resin in the present invention.

As the additive, a chelating agent, hydroxycarboxylic acid, or a reducing inorganic salt may be added, and it may be added to the water absorbent resin in such a manner that the amount thereof is preferably 10 to 5000 ppm by mass, more preferably 10 to 1000 ppm by mass, still more preferably 50 to 1000 ppm by mass, and particularly preferably 100 to 1000 ppm by mass. A chelating agent is preferable to be used.

The monomer is also preferable to contain a polymerization inhibitor. Examples of the polymerization inhibitor include such as methoxyphenol etc., and the content thereof is preferably 200 ppm or lower, more preferably 10 to 160 ppm, and still more preferably 20 to 100 ppm (relative to monomer).

(f) Polymerization Step (Crosslinking Polymerization Step)

Owing to the performance and the easiness of polymerization control, the polymerization method may be carried out by spray polymerization or droplet polymerization, but preferably, in general, it is carried out by aqueous solution polymerization or reverse phase suspension polymerization. The aqueous solution polymerization is preferably which are conventionally difficult to control polymerization or improve the coloring, and further preferably continuous aqueous solution polymerization. An especially preferable controlling method is a continuous polymerization method for producing the water absorbent resin in a huge scale of 0.5 t/h or higher, further 1 t/h or higher, still more 5 t/hr or higher, and still further 10 t/hr or higher by polymerization of an aqueous unsaturated monomer solution in one line. Consequently, the preferable continuous polymerization may include methods described as continuous kneader polymerization (e.g. U.S. Pat. Nos. 6,987,151 and 6,701,41), continuous belt polymerization (e.g. U.S. Pat. Nos. 4,893,999 and 6,241,928, and US Patent Application Publication No. 2005/215734).

In addition, in the continuous polymerization, polymerization at a high temperature starting (monomer at 30° C. or higher, 35° C. or higher, further 40° C. or higher, and particularly 50° C. or higher: the upper limit is the boiling point), or a high monomer concentration (30 mass % or higher, 35 mass % or higher, further 40 mass % or higher, and particularly 45 mass % or higher: the upper limit is the saturated concentration) can be exemplified as one preferable example.

The monomer stability is excellent in the present invention and the water absorbent resin with white color can be obtained even by the polymerization in such a high concentration and at such a high temperature, and thus the effect is significantly exhibited in such conditions. Preferable examples of high temperature initiating polymerization are described in U.S. Pat. Nos. 6,906,159 and 7,091,253 etc. In the present invention, the monomer stability before polymerization is excellent and therefore, production in an industrial scale is made easy.

The polymerization can be carried out in atmospheric air; however, it is preferable for coloring improvement to carry out the polymerization in an inert gas atmosphere of nitrogen or argon (e.g., oxygen concentration of 1% by volume or lower) and also, the monomer is preferable to be used for polymerization after the dissolved oxygen in the monomer or the solution containing the monomer is sufficiently replaced with an inert gas (e.g., less than 1 mg/L of dissolved oxygen). Even if such degassing is carried out, the monomer is excellent in the stability and therefore gelatinization before the polymerization does not occur and the water absorbent resin with higher physical properties and high whiteness can be obtained.

(g) Polymerization Initiator

A polymerization initiator to be used for the present invention can be selected properly in accordance with the polymerization mode. Examples of the polymerization initiator may include radical polymerization initiator such as a photodecomposition type polymerization initiator, a heat decomposition type polymerization initiator, and a redox type polymerization initiator. The amount of the polymerization initiator may be 0.0001 to 1% by mole preferably and more preferably 0.001 to 0.5% by mole to the monomer.

In the case of the amount of the polymerization initiator is large, coloring may possibly generate and in the case of the amount is low, it results in increase of the residual monomer. Further, in the case of a conventional color-improve agent, it sometimes causes a negative effect on the polymerization; however, in the polymerization by the method of the invention, the coloring can be improved without causing any negative effect on the polymerization (such as previous time and the various physical properties) and therefore, it is preferable.

Examples of the photodecomposition type polymerization initiator may include benzoin derivatives, benzyl derivatives, acetophenone derivatives, benzophenone derivatives, and azo compounds. Examples of the heat decomposition type polymerization initiator may include persulfuric acid salts (sodium persulfate, potassium persulfate, and ammonium persulfate), peroxides (hydrogen peroxide, tert-butyl peroxide, methyl ethyl ketone peroxide), azo compounds (2,2′-azobis(2-amindinopropane) dihydrochloride, 2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, etc.). Among these radical polymerization initiators, persulfuric acid salts, peroxides, and azo compounds can be used as a photopolymerization initiator.

Examples of the redox type polymerization initiator may include the above-mentioned persulfuric acid salts or peroxides in combination with reducing compounds such as L-ascorbic acid and sodium hydrogen sulfite. Further, combination use of a photodecomposition type initiator and a heat decomposition type polymerization initiator can also be exemplified as a preferable embodiment.

(2) Gel Shredding Step (Pulverization Step)

A hydrous gel-like crosslinked polymer obtained by polymerization (hereinafter, sometimes referred to as “hydrous gel”) may be dried as it is; however, it may be pulverized to be particulate (e.g., with a mass average particle diameter of 0.1 to 5 mm, preferably 0.5 to 3 mm) during polymerization or after polymerization with a pulverizer (kneader, meat chopper, or the like) if necessary.

From the physical property aspect, regarding the temperature of the hydrogel at the time of gel pulverizing, the hydrogel is kept or heated preferably at 40 to 95° C. and more preferably 50 to 80° C. The resin solid content of the hydrogel is not particularly limited; however, from the physical property aspect, it is preferably 10 to 70 mass %, more preferably 15 to 65 mass %, and still more preferably 30 to 55 mass %. It is optional to add water, a polyhydric alcohol, a mixed liquid of water and a polyhydric alcohol, a solution obtained by dissolving a polyvalent metal in water, or their vapor, or the like. In the gel shredding step, a water absorbent resin fine powder or various kinds of other additives may be kneaded.

(3) Drying Step

In order to accomplish a decrease in residual monomers, prevention of gel deterioration (urea resistance), and prevention of yellowing in the present invention, the drying step is carried out via the gel shredding step after completion of the polymerization. The time until the start of drying via the gel shredding step is more preferable as it is shorter. That is, after being discharged out of the polymerization apparatus, a hydrous gel-like crosslinked polymer after polymerization starts to be dried preferably within 1 hour, more preferably within 0.5 hours, and still more preferably within 0.1 hours (charged to a drier). In order to set the time within the range, shredding or drying is preferably carried out directly without carrying out a storage step for the gel after polymerization. Further, to decrease the residual monomer and accomplish low coloring, the temperature of the hydrous gel-like crosslinked polymer from completion of the polymerization to starting of the drying is controlled preferably at 50 to 80° C. and more preferably at 60 to 70° C.

The drying step provides a dried product having a resin solid content, which is calculated from a drying loss of the polymer (drying of 1 g powder or particles at 180° C. for 3 hours) in an amount controlled to be preferably 80 mass % or higher, more preferably 85 to 99 mass %, still more preferably 90 to 98 mass %, and particularly preferably 92 to 97 mass %. The drying temperature is not particularly limited; however, it is preferably in a range of 100 to 300° C. and more preferably in a range of 150 to 250° C. To satisfy both of the high physical properties and whiteness, it is preferably that the drying temperature is 160 to 235° C., more preferably 165 to 230° C. Further the drying time is preferably within 50 minutes. If the temperature or the time is out of the above-mentioned range, it may possibly result in decrease of the water absorption rate (CRC), increase of soluble matters (Extractables), and deterioration of whiteness index.

A various drying methods such as heat drying, hot-air drying, vacuum drying, infrared drying, microwave drying, drying by a drum drier, azeotropic dehydration with a hydrophobic organic solvent, high humidity drying using high temperature steam can be employed. It is preferably hot-air drying with a gas with a dew point of preferably 40 to 100° C. and more preferably 50 to 90° C.

(4) Crushing and/or Classifying Step (Particle Size and Adjustment Size after Drying)

The above-mentioned hydrous gel-like crosslinked polymer, the particle size may be adjusted after the drying if necessary. The polymer is preferably made to have a specified particle size to improve the physical properties by surface crosslinking described below. The particle size can be adjusted properly by polymerization (particularly reversed phase suspension polymerization), crushing, classification, granulation, and fine powder recovery. Hereinafter, the particle size is defined by a standard sieve (JIS Z8801-1 (2000)).

The mass average particle diameter (D50) of the obtained water absorbent resin particles in the dried step before surface crosslinking is adjusted to be 200 to 600 μm, preferably 200 to 550 μm, more preferably 250 to 500 μm, and particularly preferably 350 to 450 μm. It is more preferable as the particles smaller than 150 μm are less, and the particles are adjusted in a range of generally 0 to 5 mass %, preferably 0 to 3 mass %, and more preferably 0 to 1 mass %. Further, it is more preferable as the particles bigger than 850 μm are less, and the particles are adjusted in a range of generally 0 to 5 mass %, preferably 0 to 3 mass %, and more preferably 0 to 1 mass %. The logarithmic standard deviation (σζ) of the particle size distribution is preferably 0.20 to 0.40, more preferably 0.25 to 0.37, and particularly preferably 0.27 to 0.35. Its measurement method may be a method described in, for example, International Publication No. 2004/69915 and a method described in EDANA-ERT 420.2-02 by using a standard sieve. The particle diameter is preferably applied also to the finally obtained water absorbent resin after surface crosslinking.

In general if the particle size distribution is narrowed, that is, the upper and lower limits of the particle size are controlled to be narrow, the color becomes noticeable; however, the present invention is free from such color issue and is preferable. Accordingly, in the present invention, it is preferable to carry out a classification step to give the ratio of particles with 150 to 850 μm of 90 mass % or more, more preferably 95 mass % or more, particularly preferably 98 mass % (The upper limit is 100 mass %) or more, after drying.

The bulk specific gravity of the water absorbent resin particles is preferably 0.5 to 0.75 (g/cm³) and more preferably 0.6 to 0.7 (g/cm³). A measurement method thereof is described in detail in, for example, EDANA ERT 460.2-02. In the case where the bulk specific gravity is not satisfied, the stirring power index becomes difficult to be controlled or the physical properties may be lowered or powdering may be caused in some cases.

(5) Surface Treatment Step

The present invention provide a method for producing a polyacrylic acid (salt)-type water absorbent resin in a huge scale (particularly 1 t/hr) including a surface treatment step of adding and reacting a surface treatment agent to and with the dried water absorbent resin powder, wherein crosslinking reaction is carried out at a stirring power index of 3 to 15 W·hr/kg in a transverse type continuous stirring apparatus having stirring means including a feeding inlet and a discharging outlet of a water absorbent resin and one or more rotary shafts having a plurality of stirring discs, and heating means after the addition of the surface-crosslinking agent in the surface treatment step.

Herein, the stirring power index is defined as (stirring power index)=((power consumption of apparatus at the time surface treatment)−(power consumption at the time of idling)×average retention time)/(treatment amount per unit time×average retention time), and a water absorbent resin with high physical properties can be continuously and stably obtained even at the time of scale-up to an 1 t/hr or more large scale, based on a specified apparatus and specified parameters thereof (stirring power index).

The stirring power index can be easily calculated as described above from the power consumption of apparatus at the time of surface treatment and the power consumption at the time of idling. If this stirring power index exceeds 15 W·hr/kg, the physical properties (particularly, liquid permeability) are deteriorated and on the other hand, if it is under 3 W·hr/kg, the physical properties (particularly, water absorption against pressure) are also deteriorated. The stirring power index is more preferably in a range of 4 to 13 W·hr/kg, still more preferably 5 to 11 W·hr/kg, particularly preferably 5 to 10 W·hr/kg, and most preferably 5 to 9 W·hr/kg.

The control of stirring power index can be determined properly in consideration of adjustment of the supply amount and discharge amount of the water absorbent resin, the particle size or bulk specific gravity of the water absorbent resin, the rotation speed and shape of the apparatus, the composition of the surface treatment agent, and the retention time, and the preferable conditions will be described later.

As an apparatus, it is essential to use a transverse type continuous stirring apparatus having stirring means including a feeding inlet and a discharging outlet of a water absorbent resin and one or more rotary shafts having a plurality of stirring discs, and heating means. When it is not the apparatus, the physical properties at the time of continuous manufacture and scale-up is deteriorated.

Hereinafter, the preferable surface treatment method and control method of stirring power index will be described.

(5-1) Humidifying and Mixing Step

This humidifying and mixing step is a step of adding and mixing a surface-crosslinking agent to and with the water absorbent resin powder obtained through the polymerization step to the classification step.

(a) Surface-Crosslinking Agent

The present invention further includes a surface-crosslinking step after drying. The production method of the present invention is preferably applicable for a method for producing a water absorbent resin with high absorption against pressure (AAP) and liquid permeability (SFC) and continuous manufacture in a huge scale (particularly 1 t/hr), and particularly preferably applicable for high temperature surface-crosslinking of a water absorbent resin.

Treatment agents described in Patent Documents 1 to 19, particularly surface-crosslinking agents, are used for the surface treatment in the present invention. From the viewpoints of physical properties at the time of scale-up, covalent bonding surface-crosslinking agents are used among them, and preferably covalent bonding surface-crosslinking agents and ion bonding surface-crosslinking agents are used in combination.

(Covalent Bonding Surface-Crosslinking Agent)

Examples of a surface crosslinking agent to be employed in the present invention may include various organic or inorganic crosslinking agents, and organic surface crosslinking agents are preferably used. From the viewpoints of physical properties of obtained water absorbent resin, preferable examples to be used as the surface crosslinking agent are polyhydric alcohol compounds, epoxy compounds, polyamine compounds and their condensation products with haloepoxy compounds, oxazoline compounds (mono-, di-, or poly-)oxazolidinone compounds, and alkylene carbonate compounds. Particularly dehydration reactive crosslinking agents containing polyalcohol compounds, alkylene carbonate compounds, and oxazolidinone compounds, which require a high temperature reaction, are usable. In the case where no dehydration reactive crosslinking agent is used, the physical properties may sometimes be inferior or the difference of the effects of the present invention may sometimes be hard to be caused in some cases.

More concretely, examples are compounds exemplified in U.S. Pat. Nos. 6,228,930, 6,071,976, and 6,254,990. Examples are polyalcohol compounds such as mono-, di-, tri-, or tetra-propylene glycol, 1,3-propanediol, glycerin, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, sorbitol, etc.; epoxy compounds such as ethylene glycol diglycidyl ether, glycidol, etc.; alkylene carbonate compounds such as ethylene carbonate; oxetane compounds; and cyclic urea compounds such as 2-imidazolidinone.

(Ion-Bonding Surface Crosslinking Agent)

Further, other than the above-mentioned organic surface crosslinking agent, an ion-bonding inorganic surface crosslinking agent (crosslinking agent derived from polyvalent metal) may be used to improve the liquid permeability potential or the like. Examples usable as the inorganic surface crosslinking agent may include divalent or higher, preferably, trivalent to tetravalent polyvalent metal salts (organic salts and inorganic salts) and hydroxides. Polyvalent metals to be used are aluminum, zirconium, etc., and aluminum lactate and aluminum sulfate are usable. These inorganic surface crosslinking agents may be used simultaneously with or separately from the organic surface crosslinking agent. The surface crosslinking with polyvalent metals is exemplified in International Publication Nos. 2007/121037, 2008/09843, and 2008/09842, in U.S. Pat. Nos. 7,157,141, 6,605,673, and 6,620,889, in US Patent Application Publication Nos. 2005/0288182, 2005/0070671, 2007/0106013, and 2006/0073969.

Further, other than the above-mentioned organic surface crosslinking agent, a polyamine polymer, particularly, having a mass average molecular weight of about 5000 to 1000000 may, be used simultaneously or separately to improve the liquid permeability potential and the like. Usable polyamine polymers are exemplified in U.S. Pat. No. 7,098,284, International Publication Nos. 2006/082188, 2006/082189, 2006/082197, 2006/111402, 2006/111403, and 2006/111404 etc.

(The Use Amount)

The use amount of the surface crosslinking agent is preferably 0.001 to 10 parts by mass and more preferably 0.01 to 5 parts by mass relative to 100 parts by mass of the water absorbent resin particles. Water can be preferably used in combination with the surface crosslinking agent. The amount of water to be used is preferably in a range of 0.5 to 20 parts by mass and more preferably 0.5 to 10 parts by mass relative to 100 parts by mass of the water absorbent resin particles. In the case of using the inorganic surface cross-linking agent and the organic surface crosslinking agent in combination, the agents are used preferably in a range of 0.001 to 10 parts by mass and more preferably 0.01 to 5 parts by mass, respectively.

Further, at that time, a hydrophilic organic solvent may be used and its amount is in a range of 0 to 10 parts by mass and preferably 0 to 5 parts by mass relative to 100 parts by mass of the water absorbent resin particles. Still more, at the time of mixing a cross-linking agent solution with the water absorbent resin particles, a water insoluble fine particle powder, and a surfactant may coexist to an extent that the effect of the present invention is not hindered, that is, in a range, for example, of 0 to 10 parts by mass, preferably 0 to 5 parts by mass, and more preferably 0 to 1 part by mass. The surfactant to be used and its use amount are exemplified in U.S. Pat. No. 7,473,739 etc.

(b) Mixing Apparatus

In the present invention, for mixing the surface treatment agent, a continuous high speed rotation stirring type mixing apparatus. Especially, a transverse type continuous high speed rotation stirring type mixing apparatus (e.g., FIG. 4) is preferable used. In addition, the surface treatment agent refers to the above-mentioned surface-crosslinking agent, or a substituent thereof (e.g., a radical polymerization initiator such as a persulfuric acid salt and a monomer) and is also a concept including a solution or dispersion liquid thereof. The stirring speed is preferably 100 to 10000 rpm and more preferably 300 to 2000 rpm. The retention time is within 180 seconds, more preferably 0.1 to 60 seconds, and particularly preferably 1 to 30 seconds.

(c) Temperature of Water Absorbent Resin Before Surface-Crosslinking

In the present invention, the temperature of water absorbent resin particles (e.g., a water absorbent resin in which it is mixed with the surface-crosslinking agent and then before it is introduced into the surface-crosslinking step; the resin is also referred to as a particulate water absorbent agent) to be supplied to the surface-crosslinking step or to a transportation tube is preferably 30° C. or higher, more preferably 40° C. or higher, and still more preferably 50° C. or higher. The upper limit thereof is preferably 100° C. and more preferably 95° C. Deterioration of the physical properties of the water absorbent resin particles (particulate water absorbent agent) can be suppressed by keeping the temperature of the particulate water absorbent resin to be supplied to a transportation tube at a prescribed temperature or higher. Specifically, a significant effect is caused on maintain of the physical properties such as saline flow conductivity (SFC).

(5-2) Heat Treatment Step

This heat treatment step is a step of heating the wet mixture of a water absorbent resin powder and a surface treatment agent solution mixed in the humidifying and mixing step to cause surface-crosslinking reaction.

(a) Structure of Heating Apparatus

(Inclined Angle)

Heat treatment is carried out for the water absorbent resin after the surface treatment agent is added to the stirring apparatus. The transverse type continuous stirring apparatus (e.g., FIG. 1) is a necessary apparatus. In terms of control of the stirring power index, the transverse type continuous stirring apparatus is preferable to have a downward inclined angle of 0.1 to 10°. The inclined angle is more preferably 0.5 to 5° and still more preferably 1 to 4°. In the case where the inclined angle does not satisfy the range, the stirring power index becomes too high or too low and the physical properties of the water absorbent resin may possibly be deteriorated in some cases.

(Aspect Ratio)

The aspect ratio (length of apparatus in movement direction/width of apparatus of cross section to movement direction) of the transverse type continuous stirring apparatus is preferably 1 to 20. The aspect ratio is more preferably 1.5 to 10 and still more preferably 2 to 5. The aspect ratio is determined as the ratio of the vertical (movement direction) length and the transverse (perpendicular to the movement direction in a plane) length in the inside of the apparatus. In the case where the aspect ratio does not satisfy the range, the stirring power index becomes too high or too low and the physical properties of the water absorbent resin may possibly be deteriorated in some cases or the piston flow property in the apparatus may be sometimes worsened and the stability of the performance may be worsened in some cases.

(Scraping Blades)

The transverse type continuous stirring apparatus is preferable to have scraping blades (the scraping blades are denoted, for example, by 90 a in FIG. 2). The scraping blades are described in Patent Document 31 (JP-A No. 2004-352941). If the scraping blades are used, the stirring power index can be controlled to be low and as a result, the physical properties of the water absorbent resin can be improved.

(Average Retention Time)

In terms of control of the stirring power index within the preferable range, the average retention time of the water absorbent resin is preferably controlled to be 0.05 to 2 hours. The average retention time is more preferably 0.1 to 1 hour and still more preferably 0.2 to 0.8 hours.

The average retention time measurement for the water absorbent resin in the transverse type continuous stirring apparatus of the present invention will be described. The retention time in the apparatus (also known as heating time or reaction time in the transverse type continuous stirring apparatus) is controlled by various factors such as effective volume of the apparatus (in the case where there is a stirring shaft arranged in the transverse direction, the effective volume refers to the volume covering the upper most surface of a stirring disk as the apex), the amount of the water absorbent resin particles to be supplied, inclined angle, the rotation speed of the stirring shaft, the shape of the scraping blade, the bulk specific gravity of the water absorbent resin particles, the kind of the surface treatment agent, the height of the discharge bank installed in the discharge outlet of the transverse type continuous stirring apparatus. These factors considerably affect not only the retention time but also the stirring power index. A method of measuring the average retention time is carried out by actually operating the apparatus under the condition in which the various factors are fixed and measuring the mass of the water absorbent resin particles remaining in the apparatus after stopping the operation of the apparatus. Alternatively, the average retention time can also be determined by unsteadily introducing a substance easy to be identified (for example, a compound containing sulfur) into the charging inlet of the apparatus as a tracer substance, tracing the concentration fluctuation of the substance at the discharge outlet, obtaining the retention time distribution function, and carrying out calculation according to the retention time distribution function. As the tracer substance, for example, a water-soluble sulfuric acid salt can be used. Further, as a concentration analysis method, there is a method of tracing the concentration fluctuation by measuring the intensity ratio of the characteristic x-ray of sulfur and a monovalent cation (e.g., sodium) with EPMA, XMA, or the like in the case of a partially neutralized polyacrylic acid water absorbent resin. The retention time distribution function and the average retention time are described in detail in “Introduction of Chemical Reaction Engineering (Hannou Kogaku Gairon)”, Hiroshi KUBOTA, issued by the Nikkan Kogyo Shimbun, Ltd.

(Surface Roughness)

The inside of the transverse type continuous stirring apparatus is preferably smooth and the surface roughness (Rz) thereof is controlled to be 800 nm or lower preferably. The surface roughness (Rz) is preferably 500 nm or lower, more preferably 300 nm or lower, still more preferably 200 nm or lower, particularly preferably 185 nm or lower, and most preferably 170 nm or lower. In the case where the surface roughness (Rz) of the inside of the transverse type continuous stirring apparatus does not satisfy the range, since the friction resistance with the water absorbent resin particles becomes high, the stirring power index becomes too high and the physical properties may possibly be deteriorated.

The surface roughness (Rz) means the value of the maximum height of the surface unevenness and is defined according to JIS B 0601-2001. The lower limit of the surface roughness (Rz) is 0 nm; however, there is not so much difference in the case where it is about 10 nm, and about 20 nm is satisfactory.

From the above-mentioned viewpoint, a material for the transverse type continuous stirring apparatus is preferably stainless steel and more preferably obtained by mirror finishing. The mirror finishing suppresses damage on the water absorbent resin powder. Examples of the stainless steel to be used for the apparatus include SUS 304, SUS 316, and SUS 316L.

The surface roughness (Ra) other than the surface roughness (Rz) is also defined according to JIS B0601-2001 and a preferable value thereof is also the same as that of the surface roughness (Rz). The surface roughness (Ra) is preferably 250 nm or lower and more preferably 200 nm or lower. These surface roughness values may be measured according to JIS B 0651-2001 by a probe type surface roughness meter. The surface roughness can be applied not only for the heating apparatus but also for apparatuses before and after the heating apparatus, preferably, for a cooling apparatus, a transportation pipe (particularly, a pneumatic transportation pipe) and a hopper, and the effect of improving physical properties by surface-crosslinking can be heightened.

(Rotary Shaft and Stirring Disk)

The transverse type continuous stirring apparatus has one or a plurality of rotary shafts, preferably 2 to 10 rotary shafts, and particularly 2 shafts. Further, the number of a stirring disk (e.g., in FIG. 2) or a stirring blade may be determined properly in accordance with the size (capacity) of the apparatus, and it is preferably 2 to 100 disks and more preferably 5 to 50 disks per one shaft.

(Periodical Shielding)

In terms of physical property stability and improvement by surface-crosslinking, it is to periodically shield the stirring type mixing apparatus (preferably transverse type continuous high speed rotation stirring type mixing apparatus) and the transverse type stirring apparatus after the water absorbent resin and the surface treatment agent solution are mixed to be introduced into the transverse type stirring apparatus. The interval for the periodical shielding is preferably 0.001 to 5 minutes, more preferably 0.005 to 1 minute, still more preferably 0.01 to 0.1 minutes, and particularly preferably 0.01 to 0.05 minutes. Execution of the periodical shielding makes it possible to carry out periodic introduction of the water absorbent resin into the continuous apparatus installed downstream (introduction of the water absorbent resin into a heating apparatus from a mixing apparatus or into a cooling apparatus from a heating apparatus); that is, the introduction is turned on and off intermittently. In the case where no periodic shielding is carried out in the surface-crosslinking step, the physical properties of a water absorbent resin to be obtained may possibly be deteriorated in some cases. The shielding ratio (the ratio of the time when the water absorbent resin is shielded from the continuous apparatus installed downstream) is preferably in a range of 1 to 80%, more preferably 2 to 40%, still more preferably 5 to 30%, particularly preferably 5 to 20%, and most preferably 5 to 10%, in terms of stabilization of the physical properties (standard deviation). It is sufficient that the water absorbent resin in the above amount range (e.g., 1 t/hr or more) is fed to a next apparatus even if the periodical shielding is executed. For example, in the case of a rotary valve, the shielding interval is defined as the reciprocal number (minute) of the rotation speed (rpm), and the shielding ratio is defined as a value calculated by dividing the theoretical rotation speed (rpm) per one minute of the rotary valve needed for discharging a mixture (wet powder; a mixture of the water absorbent resin and the surface-crosslinking agent solution) to be supplied to the continuous high speed mixing apparatus (theoretical rotation speed is obtained from the volume flow rate calculated from the volume per one rotation of the rotary valve, the mass flow rate of the mixture to be discharged, and the bulk specific gravity) by actual rotation speed (rpm) of the rotary valve, and multiplying the calculated value by 100. The shielding ratio is specifically defined as a value calculated by dividing the rotation speed (rpm) per one minute of the rotary valve needed for discharging a wet powder (a mixture of the water absorbent resin and the surface-crosslinking agent) out of a mixing apparatus per unit time by the actual rotation speed of the rotary valve.

The amount of the water absorbent resin retained by periodical shielding is preferably 0 to 2 mass % and more preferably exceeding 0 and to 1 mass % relative to the treatment amount. The volume per one rotation of the rotary valve may be determined properly and it is preferably 0.1 to 0.001 [m³/lev (one rotation)], more preferably 0.2 to 0.002 [m³/lev], and still more preferably 0.1 to 0.01 [m³/lev]. In the case where the periodic shielding is carried out or even when the periodic shielding is no carried out, when the continuous apparatuses (the mixing apparatus, the heating apparatus, and the cooling apparatus if necessary) are connected, the distance from the outlet of an upstream apparatus and the inlet of an apparatus installed downstream is preferably 10 m or shorter. The distance is more preferably 5 m or shorter, still more preferably 3 m or shorter, and particularly preferably 2 m or shorter. At the time of connecting these apparatuses, the apparatuses are connected up and down; that is, the downstream apparatus is connected to a lower side of the upstream apparatus. A shielding apparatus of the water absorbent resin particles may be installed between the upstream apparatus and the downstream apparatus. The lower limit of the distance may be determined properly in accordance with the sizes of the apparatuses or in a range in which a shielding apparatus described below can be housed. In the case where the distance is too large or the apparatuses are not connected up and down, the physical properties of a water absorbent resin to be obtained may possibly be deteriorated in some cases. In the case of connecting up and down, the mixing apparatus, the heating apparatus, and the cooling apparatus if necessary may be connected up and down in this order. Connecting of the cooling apparatus may be above or beside the heating apparatus.

A gate, a valve, a damper, a rotary feeder, a table feeder, or the like is installed as a periodically shielding apparatus in a connecting part of the continuous apparatuses so that the periodical shielding can be carried out. Examples of the gate to be employed include a slide gate, a roller gate, a tainter gate, a radial gate, a flap gate, a rolling gate, and a rubber gate etc. Examples of the valve to be employed include a Howell-Bunger (fixed cone dispersion) valve, a hollow jet valve (a movable cone dispersion valve), a jet flow valve, a butterfly valve, a gate valve (a partition valve), an orifice valve, a rotary valve (a valve for opening or closing by rotating a cylinder), and a Johnson valve (a valve for opening or closing by moving a conical valve body back and forth). These shielding apparatuses may be installed in an outlet of the mixing apparatus (e.g., FIG. 4 and FIG. 5), or in an inlet of the heating apparatus (e.g., FIG. 1), or in a middle part thereof after the outlet of the mixing apparatus and the inlet of the heating apparatus are connected. The shielding and connecting of the apparatuses are preferably carried out via a valve, particularly a rotary valve, among these shielding apparatuses. The size (it refers to diameter: however, in the case where the cross section is other than a circular shape, it is converted into the diameter of a circle with the same surface area) of the valve may be selected properly and it is preferably, for example, 1 to 100 cm in diameter and more preferably 10 to 50 cm in diameter.

Each shielding apparatus is operated at less than 100% of the maximum treatment amount (kg/hr; the maximum amount of a substance which can be passed through the shielding apparatus per unit time). The operation condition is preferably 5 to 95%, more preferably 10 to 90%, and still more preferably 20 to 80%. In the case where the operation condition of the shielding apparatus is out of the above-mentioned range, the physical properties of a water absorbent resin to be obtained may possibly be deteriorated in some cases and the performance may possibly become unstable. In the case where a rotary shielding apparatus such as a rotary valve is used, the rotation speed thereof may be determined properly and it is preferably 1 to 500 rpm (rotation/minute). The rotation speed is more preferably 5 to 200 rpm, still more preferably 10 to 100 rpm, and particularly preferably 20 to 100 rpm. The maximum treatment performance of the shielding apparatus may be determined properly and it is, for example, preferably 0.01 to 20 t/hr and more preferably 0.1 to 5 t/hr.

(b) Operation Condition of Heating Apparatus

After the surface treatment agent is added to the stirring type mixing apparatus, preferably the transverse type continuous stirring apparatus, and the water absorbent resin and the surface treatment agent solution are mixed, heat surface-crosslinking treatment is carried out. The transverse type continuous stirring apparatus is an apparatus necessary for heating treatment. The water absorbent resin is treated by heat treatment and is also treated by second heat treatment if necessary, and thereafter is treated by cooling treatment. The heating temperature (heat transfer surface temperature of a jacket or the like) is 70 to 300° C., preferably 120 to 250° C., and more preferably 150 to 250° C., and the heating time is preferably in a range of 1 minute to 2 hours. The heat treatment is carried out generally by a drier or a heating furnace. The present invention can provide a water absorbent resin with high whiteness even by high temperature heating or drying with air (hot blow) which conventionally causes intense coloration.

(Filling Ratio)

It is preferable to continuously supply the water absorbent resin in such a manner that the filling ratio (volume ratio) of the transverse type continuous stirring apparatus with the water absorbent resin can be 50 to 90%. The filling ratio is more preferably 55 to 85% and still more preferably 60 to 80%. In the case where the filling ratio does not satisfy the above-mentioned range, the stirring power index is hard to be controlled, and the physical properties of a water absorbent resin to be obtained may possibly be deteriorated in some cases. The position at 100% filling ratio is the apex part of a stirring disk of a rotary shaft as described above.

It is preferable to continuously supply the water absorbent resin in such a manner that the mass surface area ratio of the water absorbent resin in the transverse type continuous stirring apparatus can be 100 kg/m²/hr or lower. It is more preferably 90 kg/m²/hr or lower and still more preferably 50 to 70 kg/m²/hr. In the case where the mass surface area ratio does not satisfy the above-mentioned range, the stirring power index is hard to be controlled and the physical properties of a water absorbent resin to be obtained may possibly be deteriorated in some cases.

Herein, the mass surface area ratio is defined by the following equation.

(Mass surface area ratio)=(Mass flow rate of water absorbent resin per unit time)/(Heat transfer area of apparatus)

In the case where the jacket surface of an apparatus trough is only for heat insulation, the mass surface area ratio is defined as follows.

(Mass surface area ratio)=(Mass flow rate of water absorbent resin per unit time)/(Heat transfer area of stirring shaft and stirring disk of apparatus)

(Rotation Speed and Reaction Time)

According to the present invention, uniform heating and mixing can be carried out by adjusting the stirring speed of the transverse type continuous stirring apparatus to 2 to 40 rpm. If it is lower than 2 rpm, the stirring becomes insufficient and on the other hand, if it is higher than 40 rpm, a fine powder tends to be generated easily in some cases. The stirring speed is more preferably 5 to 30 rpm. The retention time in the apparatus is, for example, 10 to 180 minutes and more preferably 20 to 120 minutes. If it is shorter than 10 minutes, the crosslinking reaction tends to be insufficient. On the other hand, if it exceeds 180 minutes, the water absorption performance may possibly be deteriorated in some cases.

(Pressure Reduction)

In the present invention, it is preferable to set the inside of the transverse type continuous stirring apparatus to slightly reduced pressure. “Pressure-reduced state” means barometric pressure lower than atmospheric pressure. In addition, “degree of pressure reduction relative to atmospheric pressure” means the pressure difference with the atmospheric pressure and is denoted as a positive (plus) value in the case where barometric pressure is lower than atmospheric pressure. For example, in the case where atmospheric pressure is standard atmospheric pressure (101.3 kPa), the expression that “degree of pressure reduction is 10 kPa” means that barometric pressure is 91.3 kPa. In this invention, “degree of pressure reduction relative to atmospheric pressure” may also be referred to simply as “degree of pressure reduction”. In the case where pressure is not reduced, a water absorbent resin powder may possibly flow over the air intake port of the mixing apparatus and it is thus not preferable. Dust (ultrafine particles of the water absorbent resin or inorganic fine particles used if necessary) is removed from the water absorbent resin by slightly reducing the pressure and thus it is also preferable in terms of a decrease in dust.

From the viewpoint of improvement of the effect attributed to pressure reduction, the lower limit of the degree of pressure reduction is preferably higher than 0 kPa, more preferably 0.01 kPa or higher, and still more preferably 0.05 kPa or higher. Excess pressure reduction may possibly remove even a necessary water absorbent resin powder besides dust to the outside of the apparatus and it may possibly result in a decrease in yield. Additionally, from the viewpoints of suppression of leap of a powder in the system and of suppression of excess cost for the exhaust system, the degree of pressure reduction is preferably 10 kPa or lower, more preferably 8 kPa or lower, still more preferably 5 kPa or lower, and particularly preferably 2 kPa or lower. The preferable numeral range of the degree of pressure reduction may be selected arbitrarily between the lower limit and the upper limit.

(Atmosphere)

The atmosphere in the transverse type continuous stirring apparatus may be air, or an inert gas such as nitrogen for prevention of coloration or prevention of combustion, and steam may be added properly. The temperature and the dew point are determined properly, and the atmospheric temperature (defined as the gas temperature in the upper part space of the apparatus) is preferably 30 to 200° C. and more preferably 50 to 150° C. The dew point is preferably 0 to 100° C. and more preferably 10 to 80° C.

(5-3) Cooling Step

The cooling step is a step carried out arbitrarily after the heating treatment step. In the case where a dehydration reactive crosslinking agent such as a polyhydric alcohol compound, an alkylene carbonate compound, and an oxazolidinone compound is used as the surface-crosslinking agent, it is preferable to carry out the cooling step.

A cooling apparatus used in this cooling step is not particularly limited and a transverse type continuous stirring apparatus to be used in the above-mentioned heating treatment may be used and also, a biaxial stirring and drying apparatus in which cooling water is circulated in the inside of the inner wall and other heat transmission faces and which is exemplified in the Patent Document 41 (U.S. Pat. No. 7,378,453) may be used. The temperature of the cooling water is controlled to be lower than the heating temperature in the surface treatment step and preferably 25° C. or higher and lower than 80° C. In the present invention, the surface treatment reaction by heating can be controlled by a cooling apparatus arbitrarily installed and the physical properties of the water absorbent resin can be improved. Examples of the cooling apparatus to be suitably used may include cooling apparatuses exemplified in Patent Document 41 for mechanical stirring (optionally combined with stirring by gas current) and also cooling apparatuses for stirring and mixing by stirring by vibrations and stirring by gas current in combination. Herein, it is preferable to carry out the periodic shielding for the inlet of the cooling apparatus (connected to the outlet of the heating apparatus) and further for the outlet of the cooling apparatus.

The cooling step is carried out preferably in the connected transverse type continuous stirring apparatus (e.g., FIG. 3). The stirring power index of the cooling apparatus is preferable 3 to 15 W·hr/kg, more preferably 4 to 13 W·hr/kg, still more preferably 5 to 11 W·hr/kg, particularly preferably 5 to 10 W·hr/kg, and most preferably 5 to 9 W·hr/kg. The pressure reducing is similarly carried out preferably in the case of the heating step, and the periodic shielding is also similarly carried out preferably in the case of the heating apparatus. Herein, the stirring power index (4 to 13 W·hr/kg, more preferably 5 to 11 W·hr/kg, particularly preferably 5 to 10 W·hr/kg, and most preferably 5 to 9 W·hr/kg) of the heating apparatus described above (also known as heat treatment apparatus, heater) may be the same or different from that of the cooling apparatus; however, in terms of physical properties, the stirring power index of the cooling apparatus (also known as chiller) is preferable to be smaller. The stirring power index of the cooling apparatus is preferably in a range of 0.99 to 0.25 times, more preferably 0.95 to 0.50 times, and particularly preferably 0.90 to 0.55 times as high as that of the heating apparatus.

(5-4) Others (a) Number of Surface Treatment Apparatuses

In terms of improvement of the stirring power index and physical properties, the polymerization step may be carried out preferably by continuous belt polymerization or continuous kneader polymerization and a plurality of surface treatment steps are preferably carried out in parallel for the polymerization step.

In the production method of the present invention, in terms of improvement of physical properties and stabilization, the surface-crosslinking step is carried out in 2 or more lines for 1 line of the polymerization step. The 1 line in the present invention means one system in which steps proceed from a raw material (monomer) until when a polymer gel, a water absorbent resin (including a recovered fine powder product), a particulate water absorbent agent and a final product are obtained. In the case where the system is branched into two, it is referred to as “2 lines”. In other words, “2 or more lines” means a mode in which two or more apparatuses are arranged in parallel and operated simultaneously or alternately in a single step.

In the present invention, in the case where the respective steps are carried out in 2 or more lines, the upper limit for each step is about 10 lines, especially preferably 2 to 4 lines, still more preferably 2 to 3 lines, and particularly preferably 2 lines. The physical properties of a water absorbent resin to be obtained are improved by adjusting the number of the lines within the above range. From a viewpoint that in the case where the number of lines (divisions) is large, no dividing effect is caused, and the operation becomes complicated, and also it is not economical in terms of the cost, it is particularly preferable to simultaneously operate 2 lines, that is, 2 or more of the same apparatuses (particularly two apparatuses) in parallel.

In the present invention, the polymer gel or the water absorbent resin, which is a dried product of the polymer gel, is divided into 2 or more lines in the steps after the drying step, and the ratio of amounts divided may be determined for every step without any particular limitation. For example, in the case of dividing into 2 lines, the ratio is preferably 4:6 to 6:4, more preferably 4.5:5.5 to 5.5:4.5, still more preferably 4.8:5.2 to 5.2:4.8, and most preferably 5:5. Even in the case of dividing into 3 or more lines, it is preferable that the ratio of the maximum amount and the minimum amount of n divided portions is within the above range. The dividing operation may be carried out in a continuous manner or in a batch manner and the ratio of amounts divided is defined in accordance with the average amounts for a prescribed time.

In the present invention, the number of lines in the surface-crosslinking step is not particularly limited and the number thereof may be selected arbitrarily; however, in consideration of the construction cost and running cost of a plant etc., 1 line or 2 lines are preferable and 2 lines are particularly preferable. That is, in terms of physical properties, it is most preferable that the surface-crosslinking step and preferably further the crushing step and the classification step all have 2 or more lines (the upper limit is within the above range) for 1 line of the polymerization step.

In addition, in the case where a plurality of apparatuses are installed in parallel in place of one apparatus in the present invention, the apparatuses in parallel may properly be miniaturized. Even if the treatment capacity of the apparatus is miniaturized into ½, the cost of the apparatus is not lowered to a half, however, in the present invention, installation of specified apparatuses in parallel improves the physical properties of an absorbent agent to be obtained and decreases the ratio of the product out of the specification and it is thus found that it consequently results in a decrease in cost.

US Patent Application Publication No. 2008/0227932 discloses a technique of carrying out “polymerization in 2 lines” and the latter half in one line; Patent Document 30 (US Patent Application Publication No. 2007/149760) discloses a technique of “connecting in series” of a stirring and drying apparatus and a heating apparatus for surface-crosslinking; and WO 2009/001954 discloses a technique of “connecting in series” of belt polymerization apparatuses. In contrast, in the present invention, the physical properties are improved and stabilization is accomplished more than before by “arranging (substantially the same) apparatuses in parallel” in the specified step after completion of the polymerization step for one polymerization apparatus.

(Dividing Means)

In order to carry out surface-crosslinking in 2 or more lines in the present invention, a dividing step is included, and a dividing step of a particulate hydrous gel or a particulate water absorbent resin, which is a dried product of the gel, is preferably included, and a dividing step of a particulate water absorbent resin is more preferably included.

A dividing method to be employed may be, for example, the following means (a-1) to (a-3) for the particulate water absorbent resin after drying.

(a-1) A method for dividing the particulate water absorbent resin after storage in a hopper. Preferably, a quantitative feeder for a powder is used. As the quantitative feeder, a cycle feeder or a screw feeder is used suitably.

(a-2) A method for dividing the particulate water absorbent resin during the time of pneumatic transportation to a plurality of hoppers.

(a-3) A method for dividing the particulate water absorbent resin at the time of dropping (e.g., free fall). In this case, a riffle divider, a 3-way divider, or the like having hills and dams are used for the dividing. Additionally, a JIS sample reducing and dividing apparatus (riffle divider) has a structure partitioned into a large number of small chambers in which a fed sample is distributed alternately to two directions.

A dividing method to be employed for the polymer gel after polymerization may be, for example, the following means (a-4) to (a-6) or the combination thereof, and the polymer gel is supplied to the drying step in parallel.

(a-4) A method for dividing the particulate hydrous gel obtained by a kneader or a meat chopper at the time of dropping (e.g., free fall). For the dividing, a riffle divider, a 3-way divider, or the like having hills and dams are used in the outlet of the kneader or the meat chopper.

(a-5) A method for dividing the particulate hydrous gel by a quantitative feeder.

(a-6) A method for cutting a sheet-like gel obtained by belt polymerization.

Among them, it is preferable that at least the particulate water absorbent resin after drying is divided and in order for that, the polymerization gel or the particulate dried product is divided.

A preferable value of the dividing ratio of the particulate water absorbent resin and the polymerization gel to be divided in the mode is as described above.

Among them, the means (a-1) to (a-3) are preferably employed and the means (a-1) is more preferably employed in terms of the quantitative supplying property.

(b) Hopper

In terms of the surface-crosslinking property in the present invention, a hopper is preferable to be used before and after the surface-crosslinking. The hopper to be employed is more preferably a hopper with an inverse truncated pyramidal shape, an inverse circular truncated conical shape, a shape formed by adding a square pillar with the same shape as the maximum diameter part of an inverse truncated pyramid to the pyramid, a shape formed by adding a cylindrical column with the same shape as the maximum diameter part of an inverse circular truncated cone to the cone. The material thereof is not particularly limited; however, a hopper made of stainless steel is preferable to be employed and the surface roughness thereof is within the above range preferably. A preferable hopper and the shape thereof are exemplified in PCT/JP 2009/54903 and such a hopper is recommended.

(c) Transportation of Water Absorbent Resin Before and after Surface-Crosslinking

Various kinds of methods may be employed as a method of transporting the water absorbent resin before and after surface-crosslinking, and preferably pneumatic transportation is employed. From a viewpoint that the excellent physical properties of the water absorbent resin particles and/or water absorbent resin powder can be maintained stably and the obstruction phenomenon can be suppressed, dried air is preferable to be employed as primary air and secondary air used based on the necessity (additional air for pneumatic transportation). The dew point of the air is generally −5° C. or lower, preferably −10° C. or lower, more preferably −12° C. or lower, and particularly preferably −15° C. or lower. The range of the dew point is −100° C. or higher, preferably −70° C. or higher, and it is sufficient about −50° C. in consideration of cost. Further, the temperature of the gas is 10 to 40° C. and more preferably about 15 to 35° C. The adjustment of the dew point of compressed air to be used at the time of pneumatic transportation to the above range can suppress a decrease in SFC, especially, at the time of wrapping the water absorbent resin as a product and thus, it is preferable.

Besides the use of the dried gas (air), heated gas (air) may be used. A heating method is not particularly limited and a gas (air) may be directly heated using a heat source or the transportation part or pipe may be heated to indirectly heat a gas (air) flowing therein. The temperature of the heated gas (air) is preferably 20° C. or higher and more preferably 30° C. or higher, and preferably lower than 70° C. and more preferably lower than 50° C.

A method of controlling the dew point, a gas, preferably air, may be dried properly. Specifically, examples thereof include a method using a membrane drier, a method using a cooling and adsorption type drier, a method using a diaphragm drier, or a method using these methods in combination. In the case where an adsorption type drier is used, it may be of thermal regeneration manner, a non-thermal regeneration manner, or a non-regeneration manner.

(6) Other Steps

Besides the above-mentioned steps, as required, a recycling step of the evaporated monomer, a granulating step, a fine powder removing step, a fine powder recycling step, etc. may be added. Further, in order to exhibit the effect of color stabilization over time, prevent gel deterioration, or the like, an additive described below may be used for the monomer or the polymer thereof.

[3] POLYACRYLIC ACID (SALT)-TYPE WATER ABSORBENT RESIN (1) Physical Properties of Polyacrylic Acid (Salt)-Type Water Absorbent Resin

In the case the purpose is to use the polyacrylic acid (salt)-type water absorbent resin of the present invention for a sanitary material, particularly a paper diaper, it is preferable to control at least one of the following (a) to (e), further two or more including AAP, and still more three or more by the above-mentioned polymerization and surface-crosslinking. In the case the followings are not satisfied, the water absorbent resin sometimes fails to exhibit sufficient function in form of a high concentration diaper described below. The production method of the present invention is more effective for producing a water absorbent resin attaining the following physical properties and particularly for stabilizing the physical properties (narrowing the standard deviation). That is, among the following physical properties of interest, the production method of the prevent invention is preferably applied to a method for producing a water absorbent resin having a water absorption against pressure (AAP) of 20 g/g or higher for an aqueous 0.9 mass % sodium chloride solution at a pressure of 4.8 kPa, a 0.69 mass % physiological saline flow conductivity (SFC) of 1 (×10⁻⁷·cm³·s·g⁻¹) or higher, and a water absorption under no pressure (CRC) of 20 g/g or higher, and more preferably applied to a production method within the following range to improve or stabilize the physical properties.

(a) Water Absorption Against Pressure (AAP)

In order to prevent leakage in a diaper, the water absorption against pressure (AAP) for an aqueous 0.9 mass % sodium chloride solution under a pressure of 1.9 kPa and that of 4.8 kPa is controlled to be preferably 20 g/g or higher, more preferably 22 g/g or higher, and still more preferably 24 g/g or higher as one example of means for accomplishing the surface-crosslinking and the cooling step carried out thereafter. The AAP is more preferable as it is higher; however, in terms of the balance between other physical properties and cost, the upper limit of the AAP is about 40 g/g at 1.9 kPa and about 30 g/g at 4.8 kPa. The AAP is shown as a value at 4.8 kPa unless otherwise specified.

(b) Liquid Permeability (SFC)

In some cases, in order to prevent a leakage from a diaper, the liquid permeability under pressure, which is a flow conductivity SFC (defined in U.S. Pat. No. 5,669,894) to a 0.69 mass % physiological saline flow conductivity (SFC) is controlled to be 1 (×10⁻⁷·cm³·s·g⁻¹) or higher, preferably 25 (×10⁻⁷·cm³·s·g⁻¹) or higher, more preferably 50 (×10⁻⁷·cm³·s·g⁻¹) or higher, still more preferably 70 (×10⁻⁷·cm³·s·g⁻¹) or higher, and particularly preferably 100 (×10⁻⁷·cm³·s·g⁻¹) or higher as one example of means for accomplishing the surface-crosslinking and the cooling step carried out thereafter.

In order to more effectively improve the liquid permeability, especially to improve SFC to 25 (×10⁻⁷·cm³·s·g⁻¹) or higher, the present invention is preferably applied for producing a water absorbent resin with high liquid permeability.

(c) Water Absorption Under No Pressure (CRC)

Water absorption under no pressure (CRC) is controlled to be preferably 10 (g/g) or higher, more preferably 20 (g/g) or higher, still more preferably 25 (g/g) or higher, and particularly preferably 30 (g/g) or higher. The CRC is more preferable as it is higher, and the upper limit is not particularly limited; however, in consideration of balance with other physical properties, it is preferably 50 (g/g) or lower, more preferably 45 (g/g) or lower, and still more preferably 40 (g/g) or lower.

(d) Amount of Water Soluble Components (Dissolve Amount)

The amount of water soluble components is preferably 0 to 35 mass % or lower, more preferably 25 mass % or lower, still more preferably 15 mass % or lower, and particularly preferably 10 mass % or lower.

(e) Residual Monomer

Using the above-mentioned polymerization as one example of achieving means, the amount of the residual monomer is adjusted to be generally 500 ppm by mass or lower, preferably 0 to 400 ppm by mass, more preferably 0 to 300 ppm by mass, and particularly preferably 0 to 200 ppm by mass.

(2) Other Additives

Further, in accordance with the purpose, 0 to 3 mass % and preferably 0 to 1 mass % of an oxidizing agent, an antioxidant, water, a polyvalent metal compound, a water-insoluble inorganic or organic powder such as silica and metal soap, etc. as well as a deodorant, an antibacterial agent, a polymer polyamine, pulp, and thermoplastic fibers, etc. may be added to the water absorbent resin.

(3) Purpose of Use

The purpose of use of the polyacrylic acid (salt)-type water absorbent resin of the present invention is not particularly limited; however, it is preferable to use the water absorbent resin for an absorbing article such as a paper diaper, a sanitary napkin, an incontinence pad, or the like. Particularly, the water absorbent resin exhibits excellent performance in case of being used in a high-consistency diaper (one diaper in which a large amount of the water absorbent resin is used) that conventionally has a problem of malodor and coloring derived from raw materials and particularly in the case of being used in a top layer part of an absorbent body of the absorbing article.

The content (core concentration) of the water absorbent resin in the absorbent body which may contain arbitrarily other absorbing materials (pulp fibers or the like) in the absorbing article is 30 to 100 mass %, preferably 40 to 100 mass %, more preferably 50 to 100 mass %, still more preferably 60 to 100 mass %, particularly preferably 70 to 100 mass %, and most preferably 75 to 95 mass % to exhibit the effect of the present invention. For example, in the case where the water absorbent resin of the present invention is used especially for an upper layer part of an absorbent body in the above concentration, the water absorbent resin is excellent in the dispersion property of an absorbed liquid such as urine or the like owing to the high liquid permeability (liquid permeability against pressure) and therefore, an absorbent product such as paper diaper can efficiently distribute a liquid and improve the amount absorbed by the whole of the absorbent product. Additionally, since the absorbent body keeps highly advanced white color, an absorbent product with sanitary impression can be provided.

EXAMPLES

Hereinafter, the effects of the present invention will be made apparent by way of examples; however, the present invention should not be construed in a limited way based on the description of the examples. In addition, measurement methods for AAP, SFC, and the like in the following description are as described above.

Production Example 1 Production of Water Absorbent Resin Particle (A)

A continuous production apparatus for a polyacrylic acid (salt)-type water absorbent resin was used which was obtained by connecting respective apparatuses for a polymerization step (static polymerization on a belt), a gel shredding step (pulverization step), a drying step, a crushing step, a classification step, and a transportation step between the respective steps and which could carry out the respective steps continuously. The production capacity of this continuous production apparatus was about 1500 kg per an hour. Water absorbent resin particles were continuously produced by using this continuous production apparatus.

First, an aqueous acrylic acid sodium salt solution partially neutralized at 75 mol % was prepared as an aqueous monomer solution (1). The aqueous monomer solution (1) contained 0.06 mol % of polyethylene glycol diacrylate (average n number=9) as an inner crosslinking agent relative to the total mole number. The concentration of the monomer (partially neutralized acrylic acid sodium salt) in the aqueous monomer solution (1) was 38 mass %. The obtained aqueous monomer solution (1) was continuously fed onto a belt by a constant rate pump. Nitrogen gas was continuously blown in the middle of the pipe used for the feeding and the oxygen concentration dissolved in the aqueous monomer solution (1) was adjusted to 0.5 mg/L or lower. In addition, the above-mentioned “average n number” means the average number of degree of methylene chain polymerization in the polyethylene glycol chain.

Next, sodium persulfate and L-ascorbic acid were continuously mixed by line mixing with the aqueous monomer solution (1). In this line mixing, the mixing ratio of the sodium persulfate was adjusted to 0.12 g per one mole of the monomer and the mixing ratio of the L-ascorbic acid was adjusted to 0.005 g per one mole of the monomer. The continuously mixed material obtained by line mixing was supplied in a thickness of about 30 mm to a flat plane steel belt having banks in both ends and subjected continuously to static aqueous solution polymerization for about 30 minutes to obtain a hydrous gel-like crosslinked polymer (1). The hydrous gel-like crosslinked polymer (1) was finely shredded into about 2 mm by a meat chopper with a hole diameter of 7 mm, spread on a movable porous plate of a continuous ventilating band drier in such a manner that the thickness thereof was adjusted to 50 mm, and dried at 185° C. for 30 minutes to obtain a dried polymer. Herein, the time taken from the outlet of the polymerization apparatus to the inlet of the drier was within 1 minute. The entire amount of the dried polymer was continuously supplied to a three-step roll mill, followed by crushing. The roll gaps of the three-step roll mill were 1.0 mm/0.55 mm/0.42 mm in this order from the upper side. After the crushing, the crushed polymer was classified by a sieving apparatus having metal sieving nets with 850 μm meshes and 150 μm meshes to obtain water absorbent resin particles (A) containing about 98 mass % of particles having a particle diameter of 150 to 850 μm. The CRC of the water absorbent resin particles (A) was 35 g/g and the bulk specific gravity thereof was 0.6 g/cm³.

Example 1

A water absorbent resin powder (1) was produced by using a continuous production apparatus involving a surface treatment step (wetting and mixing step, heating step, and cooling step), a particle regulation step, and a transportation step connecting the respective steps, successively from the continuous production apparatus used in Production Example 1. That is, the classification step of Production Example 1 and the surface treatment step in Example 1 were connected by the transportation step.

The water absorbent resin particles (A) were pneumatically transported to a temporarily storing hopper by pneumatic transportation (temperature 35° C. and dew point −15° C.) from the classifying apparatus and continuously supplied at 1.5 t/hr via a constant rate feeder by a high speed continuous mixing apparatus (Turbulizer/1000 rpm) and at the same time a surface treatment agent solution (1) was mixed by spraying (wetting and mixing step). The surface treatment agent solution (1) was a mixed solution containing 1,4-butanediol, propylene glycol, and pure water. The surface treatment agent solution (1) was mixed with the water absorbent resin particles (A) at a ratio of 0.3 parts by mass of 1,4-butanediol, 0.5 parts by mass of propylene glycol, and 2.7 parts by mass of pure water relative to 100 parts by mass of the water absorbent resin particles (1) to give a mixture (1), a wet powder.

The obtained mixture (1) was then surface-treated by a transverse type continuous stirring apparatus (1) having a downward inclined angle of 1°, an aspect ratio of 2.2, a paddle rotation speed of 13 rpm, two rotary shafts, and stirring disks having scraping blades, and a surface roughness (Rz) of the inner surface of 500 nm (heat treatment step). At that time, the inside of the apparatus (1) was suctioned by a suctioning gas discharge apparatus having a bag filter, and the inside pressure of the apparatus was reduced to 1 kPa. A rotary valve (periodically shielding apparatus) was installed in the inlet (connecting part with the mixing apparatus) and outlet (connecting part with the cooling apparatus) of the apparatus (1). According to a previous test, the position of a discharge bank which gave an average retention time of about 45 minutes and an average filling ratio of 75% was measured, and the discharge bank was set at the position as measured. A heating source used for the surface treatment was pressurized steam at 2.5 MPa, and the temperature of the mixture (1) in the apparatus was measured by a thermometer installed near the discharge part of the transverse type continuous stirring apparatus (1), and the steam flow rate was controlled to carry out the heating in such a manner that the temperature was adjusted to 198° C. The total surface area of the stirring disks and the stirring shafts was 24.4 m² and the mass surface area ratio calculated from the total surface area (heat transfer surface area) and the treatment amount was 61.5 kg/m²/hr. The stirring power at the time of the surface treatment was 27.8 kW, the stirring power in idling was 13.5 kW, the average retention time was 45 minutes, and the stirring power index was 9.5 W·hr/kg.

Using a similar transverse type continuous stirring apparatus, it was cooled forcibly to 60° C. (cooling step). The stirring power index was 8.5 W·hr/kg at this time.

The water absorbent resin was classified by a sieving apparatus to separate the substance under 850 μm, and the substance on 850 μm (substance not passed through 850 μm) was again crushed and mixed with the substance under 850 μm to give a water absorbent resin powder (1), as a particle size-regulated product which was the total amount of the substance under 850 μm.

The obtained water absorbent resin powder (1) had 30.5 (g/g) of CRC, 29.8 (×10⁻⁷·cm³·s·g⁻¹) of SFC, 25.2 (g/g) of AAP, and 0.68 g/cm³ of the bulk specific gravity. The standard deviation of each physical property value was CRC: 0.16, SFC: 0.48, and AAP: 0.13. Additionally, these physical property values were average values of the measurement carried out by sampling (5 points) every an hour until 5 hours were passed from the starting of the operation. The physical property values were determined in the same manner for the following Examples and Comparative Examples. The results are shown in Table 1.

Comparative Example 1

A mixture (1), a wet powder, was obtained by mixing the surface treatment agent solution (1) with the water absorbent resin particles (A) as in Example 1 (wetting and mixing step).

The obtained mixture (1) was then surface treated by a horizontal transverse type continuous stirring apparatus (2) (a downward inclined angle of 0°) having an aspect ratio of 2.4, a paddle rotation speed of 10 rpm, two rotary shafts equipped with stirring discs having scraping blades, and also 1000 nm of surface roughness (Rz) in the inner face (heating treatment step). At that time, the inside of the apparatus (2) was suctioned by a suctioning gas discharge apparatus having a bag filter, and the inside pressure of the apparatus was reduced to 1 kPa. A rotary valve (periodically shielding apparatus) was installed in the inlet (connecting part with the mixing apparatus) and outlet (connecting part with the cooling apparatus) of the apparatus (1). According to a previous test, the position of a discharge bank which gave an average retention time of about 80 minutes and an average filling ratio of 44% was measured, and the discharge bank was set at the position as measured. A heating source used for the surface treatment was pressurized steam at 2.5 MPa as in Example 1, and the temperature of the mixture (1) in the apparatus was measured by a thermometer installed near the discharge part of the transverse type continuous stirring apparatus (2), and the steam flow rate was controlled to carry out the heating in such a manner that the temperature was adjusted to 193° C. The total surface area of the stirring disks and the stirring shafts was 55.7 m² and the mass surface area ratio calculated from the total surface area (heat transfer surface area) and the treatment amount was 26.9 kg/m²/hr. The stirring power at the time of the surface treatment was 53.3 kW, the stirring power in idling was 24.4 kW, the average retention time was 80 minutes and the stirring power index was 19.3 W·hr/kg.

Using a similar transverse type continuous stirring apparatus, it was cooled forcibly to 60° C. (cooling step). The stirring power index at that time was 17 W·hr/kg.

The water absorbent resin was classified by a sieving apparatus to separate the substance under 850 μm as in Example 1, and the substance on 850 μm (substance not passed through 850 μm) was again crushed and mixed with the substance under 850 μm to give a water absorbent resin powder (2), as a particle size-regulated product which was the total amount of the substance under 850 μm. The obtained water absorbent resin powder (2) had 30.3 (g/g) of CRC, 22.1 (×10⁻⁷·cm³·s·g⁻¹) of SFC, 24.5 (g/g) of AAP, and 0.70 g/cm³ of the bulk specific gravity. Table 1 shows the analysis results.

Example 2

The same operation was carried out as in Example 1 to obtain a mixture (2), a wet powder, except that 0.01 parts by mass of an aqueous solution containing 10 mass % of polyoxyethylene (number of methoxy group: 20) sorbitan monostearate, a surfactant, was added to the surface-crosslinking agent solution (1) in Example 1. That is, the mixture (2) contained further 0.001 parts by mass of the surfactant in the mixture (1).

The obtained mixture (2) was then surface-treated by a transverse type continuous stirring apparatus (3) having a downward inclined angle of 2°, an aspect ratio of 2.4, a paddle rotation speed of 10 rpm, two rotary shafts, and stirring disks having scraping blades, and a surface roughness (Rz) of the inner surface of 500 nm (heat treatment step). At that time, the inside of the apparatus (3) was suctioned by a suctioning gas discharge apparatus having a bag filter, and the inside pressure of the apparatus was reduced to 1 kPa. According to a previous test, the position of a discharge bank which gave an average retention time of about 90 minutes and an average filling ratio of 50% was measured, and the discharge bank was set at the position as measured. A heating source used for the surface treatment was pressurized steam at 2.5 MPa, and the temperature of the mixture (2) in the apparatus was measured by a thermometer installed near the discharge part of the transverse type continuous stirring apparatus (3), and the steam flow rate was controlled to carry out the heating in such a manner that the temperature was adjusted to 193° C. The total surface area of the stirring disks and the stirring shafts was 55.7 m² and the mass surface area ratio calculated from the total surface area (heat transfer surface area) and the treatment amount was 26.9 kg/m²/hr. The stirring power at the time of the surface treatment was 32.0 kW, the stirring power in idling was 24.4 kW, and the stirring power index was 5.1 W·hr/kg.

Using a similar transverse type continuous stirring apparatus, it was cooled forcibly to 60° C. (cooling step). The stirring power index at that time was 5.2 W·hr/kg.

The water absorbent resin was classified by a sieving apparatus to separate the substance under 850 μm, and the substance on 850 μm (substance not passed through 850 μm) was again crushed and mixed with the substance under 850 μm to give a water absorbent resin powder (3), as a particle size-regulated product which was the total amount of the substance under 850 μm.

The obtained water absorbent resin powder (3) had 30.3 (g/g) of CRC, 30.2 (×10⁻⁷·cm³·s·g⁻¹) of SFC, 25.1 (g/g) of AAP, and 0.66 g/cm³ of the bulk specific gravity. Table 1 shows the analysis results. The standard deviation of each physical property value was CRC: 0.13, SFC: 0.47, and AAP: 0.11.

Comparative Example 2

A mixture (1), a wet powder, was obtained by mixing the surface treatment agent solution (1) with the water absorbent resin particles (A), as in Example 1 (wetting and mixing step).

The obtained mixture (1) was then surface treated by a horizontal transverse type continuous stirring apparatus (4) (a downward inclined angle of 0°) having an aspect ratio of 1.2, a paddle rotation speed of 10 rpm, two rotary shafts equipped with stirring discs having scraping blades, and also 500 nm of surface roughness (Rz) in the inner face (heating treatment step). At that time, the inside of the apparatus (4) was suctioned by a suctioning gas discharge apparatus having a bag filter, and the inside pressure of the apparatus was reduced to 1 kPa as in Example 1. A rotary valve (periodical shielding apparatus) was installed in the inlet and outlet of the apparatus (4). According to a previous test, the position of a discharge bank which gave an average retention time of about 20 minutes and an average filling ratio of 22% was measured, and the discharge bank was set at the position as measured. A heating source used for the surface treatment was pressurized steam at 2.5 MPa as in Example 1, and the temperature of the mixture (1) in the apparatus was measured by a thermometer installed near the discharge part of the transverse type continuous stirring apparatus (4), and the steam flow rate was controlled to carry out the heating in such a manner that the temperature was adjusted to 205° C. The total surface area of the stirring disks and the stirring shafts was 12.0 m² and the mass surface area ratio calculated from the total surface area (heat transfer surface area) and the treatment amount was 125 kg/m²/hr. The stirring power at the time of the surface treatment was 28.0 kW, the stirring power in idling was 24.4 kW, the average retention time was 20 minutes and the stirring power index was 2.4 W·hr/kg.

Using a similar transverse type continuous stirring apparatus, it was then cooled forcibly to 60° C. (cooling step). The stirring power index at that time was 1.5 W·hr/kg.

As in Example 1, the water absorbent resin was classified by a sieving apparatus to separate the substance under 850 μm, and the substance on 850 μm (substance not passed through 850 μm) was again crushed and mixed with the substance under 850 μm to give a water absorbent resin powder (4), as a particle size-regulated product which was the total amount of the substance under 850 μm. The obtained water absorbent resin powder (4) had 30.5 (g/g) of CRC, 20.5 (×10⁻⁷·cm³·s·g⁻¹) of SFC, 23.2 (g/g) of AAP, and 0.62 g/cm³ of the bulk specific gravity. Table 1 shows the analysis results.

Example 3

Water absorbent resin particles (B) were produced by changing the throughput to be 3 t/hr from 1.5 t/hr in Production Example 1, further, the same operation was carried out as in Example 1 to obtain a mixture (3), a wet powder, except that the surface treatment step (wetting and mixing step, heating treatment step, and cooling step) in Example 1 was carried out in 2 lines (two respective apparatuses using in the respective steps were arranged in parallel, and throughput was 1.5 t/hr×2) for 1 line of the polymerization step (throughput: 3 t/hr), and then a water absorbent resin powder (5) as a product was obtained. The water absorbent resin powder (5) obtained had 30.5 (g/g) of CRC, 29.5 (×10⁻⁷·cm³·s·g⁻¹) of SFC, 25.2 (g/g) of AAP, and 0.67 g/cm³ of the bulk specific gravity. Table 1 shows the analysis results.

Example 4

The same operation was carried out as in Example 3, except that the surface-crosslinking step (each one apparatus as a wetting and mixing apparatus, a heating treatment apparatus, and a cooling apparatus) was carried out in 1 line for 1 line of the polymerization step (3 t/hr).

The water absorbent resin particles (B) were continuously supplied at 3 t/hr to a high speed continuous mixing apparatus (Turbulizer/1200 rpm) and at the same time a surface treatment agent solution (1) was mixed by spraying (wetting and mixing step). The surface treatment agent solution (1) was a mixed solution containing 1,4-butanediol, propylene glycol, and pure water. The surface treatment agent solution (1) was mixed with the water absorbent resin particles (B) at a ratio of 0.3 parts by mass of 1,4-butanediol, 0.5 parts by mass of propylene glycol, and 2.7 parts by mass of pure water relative to 100 parts by mass of the water absorbent resin particles (B) to give a mixture (4), a wet powder.

The obtained mixture (4) was then surface-treated by a transverse type continuous stirring apparatus (4) having a downward inclined angle of 2°, an aspect ratio of 2.5, a paddle rotation speed of 10 rpm, two rotary shafts, and stirring disks having scraping blades, and a surface roughness (Rz) of the inner surface of 500 nm (heat treatment step). At that time, the inside of the apparatus (6) was suctioned by a suctioning gas discharge apparatus having a bag filter, and the inside pressure of the apparatus was reduced to 1 kPa. According to a previous test, the position of a discharge bank which gave an average retention time of about 45 minutes and an average filling ratio of 75% was measured, and the discharge bank was set at the position as measured. A heating source used for the surface treatment was pressurized steam at 2.5 MPa, and the temperature of the mixture (4) in the apparatus was measured by a thermometer installed near the discharge part of the transverse type continuous stirring apparatus (6), and the steam flow rate was controlled to carry out the heating in such a manner that the temperature was adjusted to 200° C. The total surface area of the stirring disks and the stirring shafts was 46.5 m² and the mass surface area ratio calculated from the total surface area (heat transfer surface area) and the treatment amount was 64.5 kg/m²/hr. The stirring power at the time of the surface treatment was 57.1 kW, the stirring power in idling was 24.3 kW, the average retention time was 45 minutes, and the stirring power index was 10.9 W·hr/kg. Using a transverse type continuous stirring apparatus having a similar shape, the water absorbent resin was forcibly cooled to 60° C. (cooling step). The stirring power index at that time was 10.1 W·hr/kg.

The water absorbent resin was classified by a sieving apparatus to separate the substance under 850 μm, and the substance on 850 μm (substance not passed through 850 μm) was again crushed and mixed with the substance under 850 μm to give a water absorbent resin powder (6), as a particle size-regulated product which was the total amount of the substance under 850 μm. The obtained water absorbent resin powder (6) had 30.1 (g/g) of CRC, 28.5 (×10⁻⁷·cm³·s·g⁻¹) of SFC, 24.8 (g/g) of AAP, and 0.66 g/cm³ of the bulk specific gravity. Table 1 shows the analysis results.

TABLE 1 Comparative Comparative Example 1 Example 1 Example 2 Example 2 Example 3 Example 4 Water absorbnet resin particle (A) (A) (A) (A) (B) (B) Feed amount [t/hr] 1.5 1.5 1.5 1.5 1.5 × 2 lines 3 Surface treatment agent solution (1) (1) (1) (1) (1) (1) 1,4-BD [Parts by mass] 0.3 0.3 0.3 0.3 0.3 0.3 PG [Parts by mass] 0.5 0.5 0.5 0.5 0.5 0.5 W [Parts by mass] 2.7 2.7 2.7 2.7 2.7 2.7 Surfactant [Parts by mass] none none 0.001 none none none Mixture (Wet powder) (1) (1) (2) (1) (3) (4) Transvers type continuous stirring (1) (2) (3) (4) (5) (6) apparatus (1) × 2 lines Inclined angel [°] 1 0 2 0 1 2 Aspect ratio 2.2 2.4 2.4 1.2 2.2 2.5 Paddle rotation speed [rpm] 13 10 10 10 13 10 Surface roughness (Rz) [nm] 500 1000 500 500 500 500 Total surface area [m²] 24.4 55.7 55.7 12.0 24.4 46.5 Mass surface area ratio [kg/m²/hr] 61.5 26.9 26.9 125 61.5 64.5 Average retention time [minute] 45 80 90 20 45 45 Average filling ratio [%] 75 44 50 22 75 75 Temperature of mixture near the [° C.] 198 193 193 205 198 200 discarge part Stirring power index [W · hr/kg] 9.5 19.3 5.1 2.4 9.5 10.9 Water absorbnet resin powder (1) (2) (3) (4) (5) (6) CRC [g/g] 30.5 30.3 30.3 30.5 30.5 30.1 AAP [g/g] 25.2 24.5 25.1 23.2 25.2 24.8 SFC [×10⁻⁷ · cm³ · s · g⁻¹] 29.8 22.1 30.2 20.5 29.5 28.5 Standard deviation CRC 0.16 0.13 AAP 0.13 0.11 SFC 0.48 0.47 Note) 1,4-BD: 1,4-butanediol/PG: propylene glycol/W: pure water Surfactant: Polyoxyethylene (20) sorbitan monostearate (added in form of an aqueous 10 mass % solution) [Parts by mass]: Amount per 100 parts by mass of water absorbent resin particles Water absorbent resin powder: Average value for 5 hours (sampling was done every one hour).

Example 5

The water absorbent resin powder (6) obtained in Example 4 was pneumatically transported by leading compressed air (dew point −15° C. and temperature 35° C.) into a pipe with a surface roughness (Rz) in the inner face of 200 nm, and wrapped. The SFC after the pneumatic transportation was 28.0 and the SFC decrease ratio was 1.8%. The bulk specific gravity was 0.67 g/cm³.

Example 6

The same pneumatic transportation as in Example 5 was carried out, except that compressed air with a dew point of 20° C. was used. The SFC after the pneumatic transportation was 27.2 and the SFC decrease ratio was 4.6%.

CONCLUSION

As shown in Table 1, it could be understood that the physical properties, particularly liquid permeability SFC of a water absorbent resin after surface-crosslinking, is 25 (cm³·s·10⁻⁷/g) or higher by carrying out at a stirring power index of 3 to 15 W·hr/kg.

It was also understood that pneumatic transportation using compressed air with a specified dew point was preferable from the results of Example 5 and Example 6.

COMPARISON WITH CONVENTIONAL TECHNIQUES

The physical properties of a water absorbent resin were improved and stabilized (reduction in standard deviation) by carrying out periodic shielding and controlling the stirring power index in this application, as compared with those in Patent Documents 1 to 41, conventional surface treatment techniques. For improvements in the apparatus, for example, techniques of using specified mixing apparatuses for mixing apparatuses for surface-crosslinking agents (Patent Documents 26 to 29) are known and also techniques of heating apparatuses for carrying out reaction of water absorbent resins and surface-crosslinking agents (Patent Documents 30 and 31) are known; however, such improvement techniques in the apparatus do not indicate the present application and correspond to comparative examples in the present invention.

INDUSTRIAL APPLICABILITY

A water absorbent resin with high physical properties, particularly, high liquid permeability is provided by continuous production in a huge scale (e.g., 1 t/hr or more).

EXPLANATION OF REFERENCE NUMERALS

-   -   10: Driving apparatus     -   20: Transverse type drum     -   30: Raw material supply port     -   40: Heat medium inlet     -   40′: Heat medium inlet     -   45′: Heat medium outlet     -   50: Water absorbent resin discharge port     -   70: Rotary shaft     -   80: Stirring disk     -   80 a: Stirring disk     -   80 b: Stirring disk     -   81: Carrier gas introduction port     -   85: Gas discharge port     -   90: Scraping blade     -   90 a: Scraping blade     -   90 b: Scraping blade     -   100: Stirring apparatus (stirring means) 

1. A method for producing a polyacrylic acid (salt)-type water absorbent resin, comprising a step of preparing an aqueous monomer solution of an acrylic acid (salt), a step of continuously polymerizing the aqueous monomer solution, a step of finely shredding a hydrous gel-like crosslinked polymer during or after polymerization, a step of drying the obtained particulate hydrous gel-like crosslinked polymer, and a surface treatment step of adding and reacting a surface treatment agent to and with the dried water absorbent resin powder, wherein crosslinking reaction is carried out at a stirring power index of 3 to 15 W·hr/kg in a transverse type continuous stirring apparatus having stirring means including a feeding inlet and a discharging outlet of a water absorbent resin and one or more rotary shafts having with a plurality of stirring discs, and heating means after the addition of the surface-crosslinking agent in the surface treatment step: wherein (stirring power index)=((power consumption of apparatus at the time surface treatment)−(power consumption at the time of idling))×average retention time)/(treatment amount per unit time×average retention time).
 2. The production method according to claim 1 further comprising forcibly cooling the water absorbent resin powder by using a transverse type continuous stirring apparatus having a stirring power index of 5 to 13 (W·hr/kg) after the heating treatment.
 3. The production method according to claim 1, wherein the surface treatment step has two or more lines for one line in the polymerization step.
 4. The production method according to claim 1, wherein a periodical shielding apparatus is installed in the inlet and/or the outlet of the transverse type continuous stirring apparatus to be used in the surface treatment step.
 5. The production method according to claim 1, wherein the transverse type continuous stirring apparatus to be used in the surface treatment step has a downward inclined angle of 0.1 to 5(°).
 6. The production method according to claim 1, wherein the transverse type continuous stirring apparatus to be used in the surface treatment step has a mass surface area ratio defined below of 100 (kg/m²/hr) or lower: wherein, (mass surface area ratio)=(mass flow rate of water absorbent resin per unit time)/(heat transfer area of apparatus).
 7. The production method according to claim 1, wherein the transverse type continuous stirring apparatus to be used in the surface treatment step has an aspect ratio of 1.5 to
 5. 8. The production method according to claim 1, wherein the transverse type continuous stirring apparatus to be used in the surface treatment step has scraping blades.
 9. The production method according to claim 1, wherein the transverse type continuous stirring apparatus to be used in the surface treatment step has a surface roughness (Rz) in the inner face of 800 (nm) or lower.
 10. The production method according to claim 1, wherein the bulk specific gravity of the polyacrylic acid (salt)-type water absorbent resin is 0.50 to 0.75 (g/cm³).
 11. The production method according to claim 1, wherein a surfactant is added to the polyacrylic acid (salt)-type water absorbent resin.
 12. The production method according to claim 1, wherein the inside of the transverse type continuous stirring apparatus is kept in slightly reduced pressure.
 13. The production method according to claim 1, wherein a heat medium for the transverse type continuous stirring apparatus to be used in the surface treatment step is pressurized steam.
 14. The production method according to claim 1, wherein the surface treatment step is carried out by adding a dehydration reactive crosslinking agent and heating treatment at 150 to 250° C.
 15. The production method according to claim 1, wherein the surface treatment step is carried out by adding an organic surface-crosslinking agent and/or a polyvalent metal salt.
 16. The production method according to claim 1, wherein the polymerization step is carried out by continuous kneader polymerization or continuous belt polymerization.
 17. The production method according to claim 1, wherein the polyacrylic acid (salt)-type water absorbent resin has 25(×10⁻⁷·cm³·s·g⁻¹) or higher of flow conductivity of physiological saline solution (SFC). 